This invention relates to the lamp arts. More particularly, this invention relates to a reflector coating and a method of preparation thereof for use in reflector lamps wherein a light source is contained in a housing having a transparent section and a reflective section, the reflective section being positioned to reflect a preponderance of generated light through the transparent section.
Reflector lamps are widely used in spot lighting, head lamps, and the like. Examples of typical reflector lamps include General Electric's PAR 38 and PAR 64 lamps. PAR is the commonly accepted acronym for “parabolic aluminized reflector.” Other commercially available reflector lamps are described in U.S. Pat. Nos. 3,010,045; 4,021,659; 4,804,878; 4,833,576; 4,855,634; and, 4,959,583.
A recent area of emphasis in reflector lamp design has been to increase energy efficiency. Energy efficiency is typically measured in the industry by reference to the lumens produced by the lamp per watt of electricity input to the lamp (LPW). Obviously, a lamp having high LPW is more efficient than a comparative lamp demonstrating a low LPW. In this regard, it is expected that governmental regulations will require a significant improvement in reflector lamp LPW in the near future.
One of the most commonly used reflector coatings is aluminum film, which is deposited on the surface of a reflector by thermal evaporation and sputtering. Manufacture costs are low and the film is stable at lamp operating temperatures over the life of the lamp. Reflectivities of the film in the visible spectrum are about 88-90%, such that PAR 38 lamps incorporating the aluminum films are able to convert about 70% of the light emitted from the lamp filament tube to luminous output.
Silver films have a higher reflectivity and are used in optics, electronics, and in lighting. For the same PAR 38 example, silver-coated lamps reflectance is about 95-98%, thus the lamps are typically convert about 80-85% of the light emitted from the lamp filament tube to luminous output, a 15% lumen gain is thus expected.
Conventional manufacturing methods for assembling lamps with aluminum films incorporate several high temperature processes, including pre-heating, tubulating, aluminizing, brazing, and sealing. In the preheating step, the reflector is heated to about 735° C. In the tubulating step, ferrules and an exhaust tube are welded to the base of the reflector. The reflector is then aluminized to provide the aluminum coating. Brazing involves the welding of the light source to the ferrules. In the sealing step, a transparent cover lens is sealed over the reflector opening. Typically, an open natural gas and oxygen flame is used to carry out many of these heating steps. The flame heats adjacent portions of the reflector to high temperatures. In sealing, for example, the reflector and coating are subjected to a temperature of around 1000 ° C. in the seal region, and around 650° C. away from the seal.
Silver films may be prepared in a similar manner to the aluminum films. However, evaporated or sputtered silver films are notoriously unstable at temperatures in excess of 200° C. Silver films are readily oxidized at the temperatures used in sealing and the optical properties of the films destroyed. Unprotected silver films are thus unsuited to lamp manufacture by such processes. Moreover, the films exhibit poor chemical resistance to sulfide tarnishing, and thus the properties of the unprotected films are destroyed on exposure to the atmosphere.
Accordingly, there is a need in this art to develop a more energy efficient reflector lamp, which maintains acceptable light temperatures, light colors, life, and compatibility with current hardware.